The invention relates to a system and method for operating a pump.
Centrifugal pump units of a smaller or average power which today are part of the state of the art are usually designed as wet runners, i.e. they comprise a split tube which seals the rotor space with respect to the stator space, in particular, from the penetration of delivery fluid. The delivery fluid located in the rotor space in particular also serves for the lubrication of the bearings carrying the rotor shaft. The pumps of this construction type have proven themselves since they do not require any seal to the region of the movable parts, the rotor space may thus be conductingly connected to the pump space.
On the other hand, from the state of the art it is also known to apply dry runners, i.e., to seal the shaft carrying the pump impeller with respect to the motor. In order here to seal the rotor space with respect to the delivery fluid in a reliable manner and over a long time, one requires complicated sealing designs which are expensive and are often prone to wear.
Although the dry runners are basically superior to the wet runners with regard to efficiency since the distance between the rotor and the stator may be reduced and the magnetic field between these components is not weakened by the split tube, the extra expense for seals and the maintenance which is also required for long-term operation is so large that at least with small or average construction sizes one almost exclusively uses wet running motors. Otherwise one must ensure the permanent lubrication of the bearing.
In order to increase the hydraulic power of such centrifugal pumps and their efficiency, it is known to allocate a frequency converter to the unit, which is connected in front of the electric motor and permits a practically infinitely high rotational speed of the motor independently of the mains frequency and voltage. With an increasing rotational speed however the fluid friction between the rotor and split tube becomes noticeable so that with this construction type an increase in the rotational speed beyond certain limits is not very useful.